Such a hydraulic steering device is known from DE 195 11 501 A1. When a vehicle equipped with such a hydraulic steering device is driven in a so called “normal mode” the two fluid meters are arranged in parallel. In this case steering of the vehicle can be effected by rotating the steering wheel over a small angle. In a so called “emergency steering mode” there is no supply of hydraulic fluid under pressure. In this case one of the fluid meters is operated as auxiliary pump. Steering of the wheels of the vehicle requires a larger rotation of the steering wheel, however, with a reduced torque. Automatic switching between the two modes can be realized by means of the pressure at the supply port and by means of a spring acting in the opposite direction.
The design known from DE 195 11 501 A1 requires a rather short cardan shaft between the two fluid meters. This gives rise to some problems, for example in connection with a relative high input torque as the cardan shaft does not have many opportunities for “moving”. This can also cause scrap problems during manufacturing, as the parts of the fluid meters must fit well together. Furthermore, a lot of special components, e.g. special housing, special bolts etc., are necessary because the hydraulic fluid must flow around the bolts, which again causes that the bolts do not guide well enough, so that guiding pins etc. must be used.
In order to overcome this problem it is known from US 2009/0199915 A1 to place the selector valve between the two fluid meters. In this steering device the selector valve means comprises a rotating valve element which is arranged between the two fluid meters. This makes the construction complicated. When the two fluid meters and the selector valve between the fluid meters are clamped together with a high force the rotating valve means is no longer able to be rotated. When the clamp forces are reduced it is difficult to have a steering device without leakages. For a leakage proof steering device parts must be produced with a rather high accuracy.
Other steering devices are known from EP 1 212 231 B1 in which the selector valve means is arranged in a housing surrounding the cardan shaft. This requires additional space for the selector valve means.